As an example, FIG. 4 shows a mobile radio communication system GSM consisting of a mobile switching center MSC/VLR, a base station controller BSC, base transmitter stations BTS and a number of mobile stations MS1, MS2, MS3, MS4. The mobile switching center MSC/VLR and the base station controller BSC are considered to form a switching apparatus SA for switching calls between the individual mobile stations. FIG. 4 is only an example of a cellular mobile radio communication system, where each mobile station MS1 . . . MS4 is served by a separate base transmitter station BTS and the mobile stations MS1 . . . MS4 are each located in different cells of the network. Of course, mobile stations may move from cell to cell and may be served by different base station transmitters BTS and different base station controllers BSC. For illustrating the background of the invention, the mobile stations MS1 . . . MS4 are assumed to be located in different cells.
As indicated in FIG. 4, in mobile radio communication systems (e.g. GSM, DECT), separate channels CC1 are used for a communication, i.e. a call, between two or more subscriber stations. These channels can differ e.g. in the carrier frequency (in FDMA systems) or in time slots within a certain carrier frequency (in TDMA systems) or they can differ in both the carrier frequency and the time slot (in FDMA/TDMA systems, e.g. GSM). Normally, a certain channel can only be used by one single call, i.e. one single communication link. However, as is indicated in FIG. 4, in order to increase the capacity of the radio communication system, channels are re-used several times in a communication network of a channel re-use type, if the subscriber stations are far enough away from each other so that they cannot significantly interfere with each other. For example, in FIG. 4, the call A between the subscriber stations MS1, MS2 and the call B between the subscriber stations MS3, MS4 both use the same communication channel CC1 and since they are served by different base transmitter stations BTS in different cells assumed to be located far away from each other, obviously no problem with interference occurs here.
As is indicated in FIG. 5, if the subscriber station MS4 moves into a cell where the communication channel CC1 is used by call A between the subscriber stations MS1, MS2, then a substantial interference between call A and call B occurs and therefore, the call B is handed over to another free channel CC2. In FIG. 5 such a switching would e.g. be made by the central switching apparatus SA which can monitor the transmission conditions for each call A, B and perform a switching if the transmission conditions deteriorate due to the two calls A, B using the same channel CC1 in close proximity. Which new channel CC2 is to ba selected in turn depends on the present transmission conditions on the new channel and this channel selection procedure is accomplished by an adaptive frequency allocation (AFA) and a dynamic channel selection (DCS).
FIG. 4 and FIG. 5 only show the hand-over procedure for a general mobile radio communication system, such as a GSM system or a DECT system. Nowadays, the fixed network operators provide their subscribers with cordless telephone sets which allow a free movement of the subscriber at home. Conventionally, such cordless telephone systems which have a limited transmission/reception range, use again separate channels with no relationship to the frequencies used for transmission by the fixed network and certainly without a relationship of channels used in a GSM system. If a subscriber uses a mobile telephone and a cordless telephone, no interference problems will be caused since the two individual systems use different channels. Since for example a DECT (Digital Enhanced Cordless Telephone) and GSM-system use different frequencies, it is unlikely, that such an interference will occur.
Advanced telecommunication systems aim at providing the subscribers with more functionalities, i.e. to use a mobile telephone registered for the GSM network also as a cordless telephone at home. That is, it would be desirable to provide the capability (and the practicability) of a cordless system which is entirely based on the GSM-technology and thus presents an alternative to a DECT system. That is, the same handy should be used in the GSM-network and then be transferred to a local base station at home using the same GSM-technology for a communication. Thus, whilst conventional handys incorporated a switching between the GSM-system and the DECT system, such a fully GSM-based system would allow the user to use the handy in the GSM-network as well as at home, only that at home it would be served by another (local) station.
In doing so, the GSM-based Cordless Telephone System CTS has to use the same frequency spectrum, i.e. the same channels, as the overlaying cellular GSM system. Furthermore, since the cordless telephone system CTS is only connected to the fixed network, it cannot be controlled by the cellular system and can therefore not be considered by the frequency or channel planning of the cellular network.
Thus, since the cordless telephone system and the GSM system must use the same channels if the mobile telephone is also to be used as a cordless telephone at home, the hand-over of a call on a channel to another channel cannot be controlled by a central unit, since the two overlaying systems (cordless telephone system and GSM system) are not centrally but separately controlled. Therefore, an autonomous channel selection procedure has to be implemented in the cordless telephone system, so as to select channels from the available frequency spectrum that can be used by the CTS without interfering with the cellular GSM system as well as other CTS users. This channel selection procedure is accomplished by an Adaptive Frequency Allocation (AFA) and a Dynamic Channel Selection (DCS). The AFA routine provides each CTS base station with an ordered set of carrier frequency candidates which minimize the interference with the cellular system, whereas the DCS routine provides the CTS base station with a certain suitable channel which minimizes the interference with other CTS subscribers and the cellular subscribers. However, since the two individual systems are separate from each other, the hand-over must be controlled separately and autonomously by each base station or mobile station being part of each communication link.